American Oil and Gas Reporter - December 2018 - 69

SpecialReport: Well Stimulation & Completion Technology
oping a long-lasting paraffin inhibitor
that is compatible with most water-based
completion fluids," he says. "It should
last a year or longer down hole, greatly
reducing the need for separate chemical
treatments."
Turning to Solvay's cementing innovations, Qu says the company is researching cement formulations that minimize
fluid loss in high temperature wells
without compromising cement slurry viscosity characteristics. "We have had great
success with first-generation formulations,
and we are refining those to increase efficiency and flexibility," he concludes.
Surfactant Evaluation
Surfactants have the potential to significantly increase recovery factors,
says Larry Tietjen, president of Verde,
a Midland-based chemical company.
"An effective surfactant will imbibe
into the formation and change it from
oil-wet to water-wet. This will separate
the oil from the formation, and allow it
to flow more easily into the wellbore,"
he explains.
Getting beyond the surface is critical
to the surfactant's long-term effectiveness,
Tietjen emphasizes. "Unfortunately, many
of the surfactants commonly used are
based on solvents. They clean the formation's surface and allow the oil there to
flow more quickly but rarely get beyond
the surface to mobilize trapped oil. As a
result, these surfactants tend to increase
initial production but have a limited effect
on the well's long-term performance."
Tietjen acknowledges that solventbased surfactants perform well in column
tests but argues that these tests do a
poor job of evaluating surfactants' effectiveness. "To perform a column test,
the lab grinds cuttings from the well,
which destroys the porosity. Without
that porosity, it is impossible to see how
well the surfactant will imbibe into the
formation to dislodge the oil," he says.
"The test will show how well a chemical
flows through compact spaces, but that
makes it more of a flowback test than a
completion test."
Tietjen adds that column tests generally
are performed at 75 degrees Fahrenheit,
far below the temperatures typically encountered down hole.
To develop a more accurate way to
predict surfactant performance, Tietjen
says, Verde worked closely with David
Schechter, a petroleum engineering professor at Texas A&M University. Schechter

came up with a test centered around the
contact angle between an oil droplet and
a rock chip that has been exposed to the
surfactant.
"This test requires a chip from a core
sample that is a fifth the size of a quarter,
which is quite small but still larger than
most cuttings," Tietjen outlines. "The lab
ages the chip for two weeks in crude oil
from the well or at least the same formation, then puts the chip in an aqueous solution containing the surfactant. Finally,
the lab places an oil droplet on the chip's
surface and measures the contact angle."
Without a surfactant, Tietjen says interfacial tension between the oil-wet chip
and the oil droplet will cause the droplet
to spread across the surface, forming a
wide sphere and yielding a high contact
angle. With an effective surfactant-one
that water-wets the chip and reduces interfacial tension-the droplet will take on
an oblong shape, with its narrow base
touching the chip. The angle formed between the oil droplet's outer slope and
the chip will be acute.
According to Tietjen, the lower the
angle, the more effective the surfactant.
"A low angle indicates the surfactant has
imbibed into the chip and rendered it
water-wet," he says. "In the field, such
surfactants penetrate the formation and
displace the oil from the surface so it can

flow into the fracture network. Because
that fracture network is water-wet, it acts
as a slick, oil-repelling highway that encourages the oil to move into the wellbore,
increasing production and ultimate recovery rates."
Field trials validate that thesis, Tietjen
assures. He cites a 17-month trial that
compared two wells targeting the Wolfcamp formation in Howard County, Tx.:
a well with a new surfactant at concentrations capable of producing 83-degree
contact angles, and a direct offset that
had the same lateral length, stage count
and artificial lift system but employed a
more traditional surfactant.
"The well with the new surfactant
produced 145,000 additional barrels of
oil," Tietjen reports. "That number is impressive, but I am more excited that the
gap between the wells' cumulative production continues to widen even after 17
months."
The surfactant has yielded similarly
impressive results in 90 completions, including many targeting the Wolfcamp
and Spraberry formations in West Texas
and Oklahoma's Caney formation, Tietjen
says. He mentions that operators plan to
deploy the surfactant in North Dakota,
Colorado and Wyoming.
"We now have a second-generation
version that has even lower contact angles.

Surfactants that can penetrate and water wet the formation signiﬁcantly increase production. Their ability to do so can be measured using contact angle tests, Verde reports. The
company says its latest surfactants are able to achieve extremely low contact angles.

DECEMBER 2018 69

American Oil and Gas Reporter - December 2018

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